1. Field of the Invention
The present invention relates to the field of fabrication of semiconductor devices. More specifically, the invention relates to the fabrication of silicon-germanium semiconductor devices.
2. Background Art
In a heterojunction bipolar transistor, or HBT, a thin silicon-germanium layer is grown as the base of a bipolar transistor on a silicon wafer. The silicon-germanium HBT has significant advantages in speed, frequency response, and gain when compared to a conventional silicon bipolar transistor. Speed and frequency response can be compared by the cutoff frequency which, simply stated, is the frequency where the gain of a transistor is drastically reduced. Cutoff frequencies in excess of 100 GHz have been achieved for the HBT, which are comparable to the more expensive GaAs. Previously, silicon-only devices have not been competitive for use where very high speed and frequency response are required.
The higher speeds and frequency response of the HBT have been achieved as a result of taking advantage of the narrow band gap for silicon-germanium. The energy band gap of silicon-germanium is smaller than it is for silicon, lying between the intrinsic band gap of silicon (1.12 eV) and germanium (0.66 eV). The band gap is reduced further by the compressive strain in the alloy layer, with the band gap being reduced even further with increasing germanium content. The narrower band gap helps to increase the gain of the HBT by facilitating carrier injection across the emitter-base junction.
It is also known in the art that grading the concentration of germanium in the silicon-germanium base builds into the HBT device an electric field, which accelerates the carriers across the base, thereby increasing the speed of the HBT device compared to a silicon-only device. A reduced pressure chemical vapor deposition technique, or RPCVD, used to fabricate the HBT device allows for a controlled grading of germanium concentration across the base layer. As already noted, speeds in the range of approximately 100 GHz have been demonstrated for silicon-germanium devices, such as the HBT.
Because the benefits of a high gain and high speed silicon-germanium HBT device can be either partially or completely negated by a high base contact resistance, it is important that the resistance of the base contact be kept to an absolute minimum. By way of background, a base contact may be provided by forming an electrical conductor in contact with the epitaxial silicon-germanium base region. Such a conductor is usually composed either of a metal or polycrystalline semiconductor material. The choice of material is driven by several constraints and considerations. For example, it is not feasible to use a metal early in the fabrication for fear of contamination as well as practicality of integration. Also, the geometry of the base region may necessitate a contact of semiconductor material rather than metal. As such, it is often required to form a contact made of polycrystalline silicon-germanium to make an electrical connection with the single crystal silicon-germanium base. The process of fabricating the epitaxial or vertical transistor profile while simultaneously fabricating the poly-crystalline external base contact is known as a non-selective process. In other words, two critical components of the HBT structure are fabricated concurrently. Just as the physical properties of the single crystal silicon-germanium base are important in building a high-performance silicon-germanium HBT, attaining the optimum physical properties of the polycrystalline silicon-germanium material to serve as the external base contact is equally important to realize the inherent performance benefit offered by the HBT device.
In order to achieve satisfactorily low resistance in the base contact, it is required to control the thickness and morphology of the base contact polycrystalline material. At the same time, the processes that are used to control the attributes of the polycrystalline base contact material must not deleteriously affect the properties of the epitaxial base itself. Accordingly, a fabrication technique is needed to achieve independent control of the physical properties of the polycrystalline base contact, including thickness and morphology, while maintaining the physical properties of the epitaxial silicon-germanium base, in order to allow the formation of an optimum low-resistive conduction path to the base of the HBT.
According to one known technique of fabrication of heterojunction bipolar transistor devices, a relatively high processing temperature is used for silicon-germanium epitaxy in an RPCVD technique. The high temperature—higher than approximately 700° C.—sacrifices important control over the thickness and morphology of the base contact and, consequently, the resulting base contact resistance.
Thus, there is need in the art to retain important control over the resulting base contact resistance without sacrificing manufacturing throughput. There is further need in the art to decrease the base contact resistance of a heterojunction bipolar transistor device produced in the fabrication process while maintaining the desired dopant and germanium concentration profiles in the HBT. There is also need in the art to maintain the high throughput in the fabrication process when reducing the base contact resistance and while maintaining the desired dopant and germanium concentration profiles in the silicon-germanium base.